Spin transfer (spin torque) devices are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high, and if the dimensions of the multilayer are small. Spin transfer devices also known as spintronic devices wherein at least one of the ferromagnetic layers in a magnetoresistive (MR) junction has perpendicular magnetic anisotropy have an advantage over devices based solely on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is a key challenge for future MRAM (Magnetoresistive Random Access Memory) applications and other spintronic devices such as microwave generators.
A spin transfer oscillator (STO) is a magneto-resistive (MR) thin film device which can have an induced RF frequency magnetization oscillation within at least one of its magnetic layers by applying an electrical current. As described in U.S. Pat. No. 7,616,412, an STO may be used as a high Q factor RF signal generator if the oscillating magnetization is transformed into resistance fluctuations through a MR effect. An STO comprises at least three layers including a magnetic oscillating layer (MOL), a magnetic reference layer (MRL), and a non-magnetic spacer sandwiched between the MOL and MRL. These three layers may be considered a junction. When electrons transit the MRL and become polarized, the polarized electrons then pass through the non-magnetic spacer and through the MOL to induce a gyromagnetic oscillation also known as ferromagnetic resonance (FMR) in the MOL. A PSTO (perpendicular spin torque oscillator) is a version of an STO wherein the MRL has perpendicular magnetic anisotropy (PMA) and is magnetized in a direction perpendicular to planes of the junction layers. With a perpendicular magnetization of the MRL, a full amplitude in-plane oscillation of the MOL can be achieved.
Referring to FIG. 1, a PSTO structure is depicted from U.S. Pat. No. 7,616,412 and includes MRL 1 that has intrinsic anisotropy which keeps its magnetization perpendicular to the film plane, non-magnetic junction layer 2, and MOL which is a composite comprised of layers 31-33. Layers 31, 33 are soft magnetic layers and middle layer 32 has a perpendicular anisotropy 34 normal to the film plane that biases layers 31, 33 through exchange coupling. Layers 1, 2, and 31-33 constitute the PSTO component 11 of the stack. There are other layers 4-8 above the MOL 3 for sensing the magnetization oscillation of the MOL. In particular, second junction layer 4, reference layer 5 with in-plane magnetization, non-magnetic exchange layer 6 typically made of Ru, and pinned layer 7 are sequentially formed as a second stack 12 on PSTO stack 11. Layers 5-7 comprise a generic synthetic anti-ferromagnetic (SAF) configuration commonly used in commercial giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR) sensors. Anti-ferromagnetic layer 8 pins reference layer 5 and pinned layer 7 through exchange coupling.
During an operating mode, electrons flow through the entire stack from bottom electrical contact 10 to top electrical contact 9, and MRL 1 magnetization is oriented in the opposite direction to that of MOL layers 31-33. As a result of the spin torque effect, electrons passing through the junction layer 2 excite MOL layers 31-33 magnetization from a quiescent near vertical state into an oscillation state. PSTO 11 is intrinsically a MR junction in which relative magnetization angle change between layers 1 and 31-33 will produce a resistance change across stack 11 that can be measured as a voltage signal when a current flows through the stack. However, when MOL layers 31-33 reach a stable magnetic oscillation with a significant amount of in-plane magnetization component, the relative angle between the magnetizations of MRL 1 and MOL 31-33 does not really change which makes it difficult to generate an electrical signal through the resistance change of the MR junction to reflect the MOL oscillation. For RF voltage signal generation purposes, the prior art utilizes SAF and AFM layers above MOL 31-33 where reference layer 5 serves to generate a MR resistance change during MOL magnetization oscillation. Therefore, layers 33, 4, and 5 form another MR junction wherein the relative magnetization angle change between MOL layer 33 and reference layer 5 produces an effective resistance. As current flows between contacts 9 and 10, a voltage signal reflecting MOL layer magnetization oscillation can be produced across the entire stack.
However, there are disadvantages associated with the prior art as pictured in FIG. 1. First of all, since MOL layer 33 is used both as part of the oscillation layer in the lower MR junction and as a signal layer for the top MR junction, there is a continuous current across the two junctions. For efficient STO induced MOL oscillation, layer 2 is preferably metal to result in a lower junction based on a GMR (giant magnetoresistive) effect in which resistivity is generally quite low and current density is relatively high in order to induce MOL oscillation. On the other hand, for high signal output, the top junction involving layer 4 is preferably a TMR (tunneling magnetoresistive) element having a resistivity that is substantially larger than that of the lower GMR junction. As a result of the high current density required to excite MOL oscillation, the top TMR junction will be at a voltage drop exceeding its break down voltage. In other words, a GMR junction and a TMR junction cannot be used in the same device in a configuration where a high density current flows through both structures. This dilemma significantly limits the FIG. 1 structure applicability for actual STO RF signal generation.
Secondly, the spin torque effect from the top MR junction that is comprised of layers 33, 4 and 5 for sensing the MOL 31-33 oscillation interferes with the spin torque effect of the bottom junction of the STO stack 11 so that MOL oscillation quality is degraded by the presence of the SAF layers 5-7 and AFM layer 8. Simulation data is available to support this fact and is presented in FIGS. 2a-2b, and FIGS. 3a-3b. 
Referring to FIGS. 2a-2b and FIGS. 3a-3b, micro-magnetic simulations are shown for a PSTO comprised of stack 11 only (FIGS. 2a, 3a) and for a PSTO that includes all layers 1-8 in FIG. 1 as depicted in FIGS. 2b, 3b. FIG. 2a is the oscillation time trace of the in-plane magnetization component for MOL in stack 11 only, and FIG. 2b is a similar oscillation time trace for a PSTO having all layers in FIG. 1. FIG. 3a and FIG. 3b are the corresponding power-spectrum-density (PSD) plots of the time traces in FIGS. 2a, 2b, respectively, where the FMR peak represents the frequency and power of the oscillation. When a top junction, SAF and AFM layers are included in a PSTO stack as represented in FIGS. 2b, 3b, MOL oscillation shows irregular behavior and lower power than in the case of PSTO stack 11 only. Therefore, although the additional layers 4-8 are useful in acquiring oscillation information of the MOL 31-33, the second (top) MR junction also changes the MOL oscillation behavior. The change to irregular oscillations and lower power is especially not desirable when uniform and high power MOL oscillation is required on a continuous basis for optimum performance while characterization of oscillation frequency is only needed occasionally, for example, when the PSTO is used as a RF field generator in a magnetic recording device.
Another disadvantage of the FIG. 1 structure is that the PMA layer 32 is separated from the lower junction layer 2 by a soft magnetic layer 31. This configuration is found not to be effective for generating STO induced oscillation in MOL 31-33 in actual fabricated devices as will be explained later with regard to FIG. 5b. 
U.S. Patent App. Publication 2009/0201614 discloses a hybrid spin torque oscillator having a separate oscillating field generating unit that supplies an oscillating field through magnetostatic coupling to a magnetoresistive (MR) element. When a DC current is applied to the MR element in the presence of the oscillating field, magnetic resonance occurs in the MR free layer. An AC component is formed by device resistance variation as a function of time and is extracted by a bias tee formed with a capacitor and an inductor to obtain a microwave output.
U.S. Pat. No. 7,589,600 describes the use of an electromagnet to provide an in-plane field that induces an oscillation in a STO structure.
In U.S. Pat. No. 7,009,877, a three terminal structure is employed in spin torque switching of a MRAM storage layer wherein the switched state is sensed with a MTJ.
U.S. Patent Application 2010/0110592 discloses a STO having a non-magnetic layer disposed between a first magnetic layer and a second magnetic layer. A magnetic field is applied in a direction substantially perpendicular to the principal plane and a current is passed perpendicular to the principal plane.
U.S. Pat. No. 7,652,915 describes spin torque microwave oscillation wherein the oscillation frequency is a function of the memory element size, shape, and anisotropy. The memory element is read by measuring resistance either with a DC current or by measuring the resonant frequency.
U.S. Pat. No. 7,764,538 teaches vertical current flow from an oscillator to a MTJ.
In U.S. Patent Application 2009/0310244, an electromagnetic field generating element is disclosed that comprises a spin wave excitation layer adjacent to a first magnetic pole and having its magnetization direction varied in response to external magnetic fields. A spin wave excitation current flows perpendicular to the layer planes from the first magnetic pole to a second magnetic pole.